The demands for ever-increasing bandwidths in digital data communication equipment at reduced power consumption values are constantly growing. These demands not only require more efficient integrated-circuit components, but also higher performances, interconnect structures and devices. Indeed, as one example, the International Technology Roadmap for Semiconductors (ITRS) projects that high performance chips in the very near future will have operating frequencies, both on-chip and off-chip, rising above 50 GHz. Conventional metal-wire based interconnects have played a central role in the microelectronics revolution. It is apparent that wire-based interconnect devices will be challenged to enabling even higher operating frequencies.
However, besides challenges with regard to bandwidth, the wire-based interconnect of the future may struggle significantly with a high power consumption. The power requirement of electronic components typically increases with increased bandwidth, which in some cases results in increased cooling requirements which further increases the power consumption of the electronic system as a whole. The power and cooling requirement may be particularly challenging to meet in data centers where larger quantities of servers are pooled often closely spaced. Such pooling inherently requires large quantities of interconnects which therefore may add significantly to the power and cooling requirements of the datacenter.
One approach to solve this problem includes utilizing optical interconnects as an alternative to wire-based interconnections, as optical fibers have a significantly higher bandwidth relative to an electrical wire. In one embodiment it is therefore an object of the present invention to provide means for reducing the power requirement of an optical interconnect.
An optical interconnect is typically composed by a transceiver module in each end adapted to transmit optical information along one or two optical fibers. The transmitter of each transceiver typically comprises a driver circuit coupled to a light source and a receiver circuit coupled to a photo detector. Typically optical fibers are used as transmission medium in which case the light source and photo detector will be coupled to fibers. A driver circuit (often located on a driver chip) is a circuit tailored to generate a waveform appropriate to drive a light emitting device in response to an input signal which is typically a binary data stream. The combination of a driver circuit and a light source is referred to as a transmitter. A receiver circuit (often located on a receiver chip) is a circuit tailored to receive the output from the light detector and generate a corresponding binary data stream. The combination of a receiver circuit and a photo detector is referred to as a receiver. Often the receivers and transmitters provide multiple channels, i.e. the ability to transmit or receive via multiple light sources or photo detectors. Sometimes driver and receiver circuits are combined on the same chip which is then referred to as a transceiver chip. Besides driver, receiver and/or transceiver chips optical modules may comprise further chips and electronics such as e.g. a microcontroller. Typically, the binary signal used in such optical links is an amplitude modulated NRZ signal but other signal types are in principle possible.
In a typical optical interconnect Vertical Cavity Surface Emitting Laser (VCSEL) diodes are utilized as light emitter to transmit binary data over optical fibers. However, the light source may in principle be any suitable light source and the transmitted waveform may be any suitable waveform for transmitting information. Most light emitters have a threshold current above which they substantially begin to emit light. Increasing the current driven through the emitter from zero to above said threshold may be time consuming, and therefore a bias current is typically driven through the light source. Often the bias current is set just below, at the threshold or above the threshold, but it may also be set to be well above threshold. This bias current is often programmable so as the same circuit design may be utilized to drive different light emitters and/or be used for different applications. Additional time varying current which modulates the emission from the light emitter is referred to as the modulation current.
For a VCSEL the suitable modulation and bias current levels change with temperature and age so the driver is often programmed with VCSEL characteristics that describe the performance as a function of temperature. The actual operation of the driver is then often determined by the temperature and the programmed levels.
To ensure stable operation it is often necessary to characterize each VCSEL individually, or if the uniformity from VCSEL to VCSEL is sufficiently good it is necessary to characterize the VCSEL on a batch level to determine suitable bias and modulation currents. One of the main problems with the manufacturing of VCSEL is that the uniformity from batch to batch may be poor and therefore it may not be possible to use the same programming for all VCSEL's. In addition, the bias and modulation currents change with the age of the VCSEL, but most systems do not have a method for compensating for aging of the VCSEL. Some systems overcome this by a build in timer adding further complexity to the system. Again aging may not be well characterized for VCSEL's and may have large batch to batch variations. This also means that it may not be possible to have a reliable end of life warning, at least not without a large safety margin.
As outlined above, the response of a light source in the transmitter, such as a VCSEL, may vary due to factors such as temperature, aging and production variations. It may therefore be valuable to monitor the optical signal received and/or transmitted for example to monitor the quality of the signal to allow prediction of a bit error rate. While it is relatively straight forward to measure the average optical power it is often more challenging to determine the modulation amplitude. One method is to apply one or more peak detector(s) to determine at least one of the logic levels of the binary signal i.e. the low level and/or the high level. By knowledge of either or both logic levels or one logic level and the signal average, the modulation amplitude of the signal and/or the extinction ratio may be determined. A measure of the logic levels may also be applied to set the decision threshold level for a receiver such as in U.S. Pat. No. 5,371,763 where peak detectors are applied to determine the logic-low level and logic-high level from the preamble in a burst mode communication system. However, the inventors have found that the response time of a peak detector circuit is often related to the power consumption of said circuit so that the power consumption of such a circuit can be relatively high due to high communication speeds current implemented and expected in the future. The inventors has also identified that the response time must often be chosen carefully to ensure a useful measurement. Too fast and the peak detector may determine the peak height of sub features in the signal such as peaking during transitions between low and high. Too slow and the measure peak level may be incorrect due to averaging. Hence alternative methods of determining the amplitude of a binary signal in an optical transmitter and/or receiver is needed.